This invention relates to a method for sealing a thermoplastic cap to the neck of a thermoplastic container using an ultrasonic welding apparatus. More specifically, the invention concerns the sealing of a thermoplastic flanged cap to the neck of a thermoplastic container, the flange of the cap being superposed on the neck and having a lateral edge which is extended beyond the periphery of the neck. The extended flange is reshaped by a cavity disposed in an ultrasonic resonator to provide a hermetically sealed capped container. Quite specifically, the invention concerns the dimensioning of the ultrasonic resonator forming a portion of the ultrasonic welding apparatus for accomplishing the sealing of the type stated above.
Ultrasonic sealing of caps to containers is well known in the art. For example, U.S. Pat. No. 3,494,817, issued to A. E. Whitecar, entitled "Rotary Turret Sonic Welding Apparatus", dated Feb. 10, 1970, discloses a cap having a flange for being welded to the upper surface of the neck of a container using an ultrasonic welding apparatus. The flange is dimensioned to be of the same diameter or of a slightly smaller diameter than the diameter of the neck. While the apparatus has been successful in joining a cap to a container, there have been instances where a non-hermetic seal is formed. The present invention provides an apparatus for sealing a cap of similar shape to the neck of a container while providing greatly improved reliability in forming such a seal.
In accordance with the present invention, the flange of the cap is dimensioned to be slightly larger than the diameter of the neck of the container. For sealing, an ultrasonic resonator is used which is provided with a domical cavity extending axially from the output surface of the resonator toward the input surface, the maximum opening of the cavity being disposed at the output surface and being at least equal to the diameter of the container neck. As the resonator is urged into intimate contact with the flange of the cap and ultrasonic energy is applied, the periphery of the flange begins to melt and soften. As the engagement force is continued between the flange and container, the container neck likewise begins to soften and melt under the influence of the ultrasonic energy which manifests itself in a combined longitudinal and shear direction. The extended flange material becomes trapped within the domical cavity of the resonator and conforms to the shape of the cavity. When the ultrasonic energy ceases and the softened and melted material solidifies while the engagement force is maintained, the cap flange becomes welded to the neck of the container. The excess material which has been trapped in the cavity forms a bead along the weld line, thus assuring a hermetic seal.
A principal object of the present invention is, therefore, the provision of an ultrasonic resonator having a cavity disposed in its output surface for use in connection with an ultrasonic welding apparatus for forming a hermetically sealed container.
Another object of the invention is the provision of an ultrasonic resonator having a domical cavity extending axially from the output surface of the resonator toward the input surface, the cavity being in the shape of a thermoplastic flanged cap to be joined to a thermoplastic container neck and having a maximum opening at the output surface at least equal to the dimension of the container neck.
Further objects of the invention will become more clearly apparent when the following description is read in conjunction with the accompanying drawings.